Well Cleaning Device

ABSTRACT

A tool for cleaning a casing of an oil well may include an elongate housing configured to be lowered into a well casing. A discharge head may be disposed at one end of the housing, the discharge head including a first electrode having a first end surface, a first insulator surrounding the first electrode, a surface of the first insulator substantially flush with the first end surface, and a second electrode having a second end surface separated from the first end surface by a discharge gap. An energy reservoir and a switch may be disposed within the housing, the switch configured to selectively couple the energy reservoir to the first electrode and the second electrode to cause energy stored in the energy reservoir to discharge across the discharge gap.

RELATED APPLICATION INFORMATION

This application is related to patent application Ser. No. 13/604,562, entitled WELL CLEANING METHOD, filed on Sep. 5, 2012.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.

BACKGROUND

1. Field

This disclosure relates to a tool for cleaning well bores, and in particular casings of oil wells, geothermal energy wells, and other wells.

2. Description of the Related Art

Petroleum products such as oil and natural gas are commonly produced by drilling a borehole or wellbore into the earth through the oil or gas producing subsurface formation. In some cases, the petroleum product may be extracted directly through the drilled borehole. More commonly, a pipe casing is placed in the borehole, for example to prevent collapse of the borehole or to prevent contamination of other subsurface formations. In the oil production industry, a distinction is commonly made between a “casing” (a pipe string that extends to the top of the borehole) and a “liner” (a pipe string, hung within a larger casing, that does not extend to the top of the borehole). This distinction is not relevant in this patent, and the term “casing” as used herein refers to any pipe string within a borehole.

The annular space between the outside of the casing and the borehole may be filled, in whole or in part, with cement to retain the casing in position and to prevent fluids from traveling between subsurface layers via the annular space. An appropriate portion of the casing may be perforated to allow the petroleum product to flow from the producing formation into the casing. In some wells, the annular space between the outside of the perforated portion of the casing and the borehole may be filled with gravel. In this patent, the combination of the borehole, the casing, the cement, the gravel if present, and any associated surface equipment will be referred to generally as a “well”. Similar wells may be used to extract superheated water for geothermal power generation or to inject water or other fluids into a subsurface formation to simulate oil or gas production.

After a period of production of fluids from a well or injection of fluids into a well, the perforations or openings in the casing may become plugged or encrusted, restricting the flow of fluids into or out of the casing. Materials that may be deposited in the casing include paraffin, asphalt, other petroleum products, mineral scale, and biological organisms. Unchecked, such deposits may reduce the flow of fluids until the well is not useful for its intended purpose, necessitating re-perforating or replacing portions of the casing.

A number of approaches have been suggested for cleaning flow-restricting deposits from wells. These approaches include treatment with acids or other chemicals, ultrasonic vibrations, or mechanical shock waves resulting from, for example, detonation of gases or explosives within the well bore. Such well cleaning techniques have limited effect and/or risk erosion or other damage to the well casing.

Another proposed technique for cleaning wells is to use repetitive electrical discharges to produce mechanical shock waves. Electric discharge devices, commonly called “sparkers”, have been used to generate acoustic waves for subsea surface mapping. Such devices create a shock wave by discharging stored energy between a pair of electrodes immersed in the body of water being mapped.

U.S. Pat. No. 4,343,356 describes a high energy electric discharge device designed to be lowered into a well casing. The device is discharged at intervals as it is lowered into the well casing to create shock waves to clean the adjacent portions of the casing. Each electrical discharge may also generate ultraviolet light and/or ozone, which may also contribute to cleaning the adjacent portions of the casing of organic or biological materials.

U.S. Pat. No. 4,343,356 teaches that an electric discharge device may be used in any natural fluid within the well casing, including water, brine, oil, solvents, acids, or other chemicals adapted to attack plugging materials. In order to avoid a random timing of and an unpredictable path for the discharges, the '356 patent describes initiating the electric discharge with a fine wire bridging the electrodes. Since the wire is vaporized by each discharge, this approach requires a mechanism for replacing the fine wire before each subsequent discharge. Providing a mechanism to feed wire between the electrodes substantially complicates the design of and may reduce the reliability of a well cleaning tool.

Further, experiments conducted by the inventors of the well cleaning method, system, and tool described herein have shown that discharging a cleaning tool in an environment that is predominantly oil results in prolonged limited-current discharges that do not produce substantial shock waves and are ineffective for well cleaning. Further, discharging a cleaning tool when oil is present in the discharge head of the tool results in rapid deterioration of insulating surfaces of the tool exposed to the discharges. This deterioration commonly takes the form of erosion or cracking along the insulating surfaces. The cause of the deterioration may be deposition of carbon on the insulating surfaces, which provides a path for the stored energy to discharge across the insulating surface.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a well bore cleaning system in an environment.

FIG. 2 is a perspective view of a well bore cleaning tool.

FIG. 3 is perspective view of a discharge head portion of the well cleaning tool.

FIG. 4 is a cross-section view of the discharge head.

FIG. 5 is an enlarged cross-section view of a portion of the discharge head.

FIG. 6 is a perspective view of a mounted electrode.

FIG. 7 is a block diagram of a well bore cleaning system including the well cleaning tool.

FIG. 8 is a flow chart of a process for cleaning a well bore.

Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number where the element is first introduced, and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

Referring now to FIG. 1, a well cleaning system 100 may include a surface installation 102 (illustrated in FIG. 1 as a truck), a cable 104, and a well cleaning tool 110. The surface installation 102 may use the cable 104 to lower the well cleaning tool 110 into a borehole 180 which is typically lined with a casing 182. All or portions of the annular space between the borehole 180 and the casing 182 may be filled with cement and/or gravel (not shown in FIG. 1). The well cleaning tool 110 may be configured to create an electrical discharge between a pair of electrodes. The electrical discharge may, in turn, generate a shock wave in the fluid within the casing 182 to clean a portion of the casing proximate to the discharge location. The electrical discharge may also produce ozone and/or ultraviolet light which may provide an additive cleaning effect. The well cleaning tool 110 may create electrical discharges at intervals as the tool is lowered into and/or withdrawn from the casing 182.

A well may extract fluids from, or inject fluids into, a subsurface layer or formation, commonly called a “production zone”. A thickness of a production zone may range from less than 25 feet to 600 feet or more. A production zone may be located at a depth of several hundred feet to several miles below the surface. Many wells extract fluids from, or inject fluids into, a single production zone. In this case, only a portion of the casing 182 passing through the production zone may have perforations or other openings to allow fluids to be exchanged between the casing 182 and the surrounding subsurface formation. Since these perforations may be most susceptible to clogging and/or plugging, the well cleaning tool 110 may be used to clean only a target portion 184 of the casing having perforations. Other wells may extract fluids from two or more production zones. Such wells may have a corresponding number of target portions 184 requiring cleaning.

As previously discussed, discharging the well cleaning tool 110 in an environment that is predominantly oil is ineffective for cleaning and causes rapid deterioration of the tool. For this reason, the one or more target portion 184 of the casing may be filled with water, or mostly water, during cleaning. Many wells produce oil mixed with water. In such wells, stopping the flow of fluids from the well may cause gravity separation of the water and oil such that a lower portion 186 of the well becomes filled primarily with the heavier water and an upper portion 188 of the well becomes filled primarily with the lighter oil. The primarily water-filled lower portion 186 and the primarily oil-filled upper portion 188 may meet at an oil-water boundary 190. The oil-water boundary 190 may be a transition zone containing emulsified oil and water rather than a sharp demarcation.

In order to prevent deterioration of the well cleaning tool 110, the oil-water boundary 190 should be above the one or more target portion 184 of the casing to be cleaned. If the amount of naturally occurring water in the well is not sufficient to cause the oil-water boundary 190 to be above the one or more target portion 184 to be cleaned, a pump 192 may be used to add additional water to the casing from a reservoir 194 or other source. Oil wells commonly have the capability of pumping water or other fluids into the casing. Water may be added to the well until the oil-water boundary 190 is above all of the one or more target portion 184 to be cleaned.

Referring now to FIG. 2, the well cleaning tool 110 may have an elongate generally cylindrical body 212 configured to be lowered into a well casing. In this patent, the term “cylindrical” means shaped as a right circular cylinder, and the term “generally cylindrical” means a large portion of an object is cylindrical within reasonable manufacturing tolerances. The term “generally cylindrical” does not preclude changes in the diameter of an object or occasional deviations from a cylindrical shape. The body 212 may have a smaller diameter than the smallest well casing to be cleaned using the well cleaning tool 110. A first end 214 of the body may be adapted to connect to a cable that provides electrical power to the well cleaning tool 110 and provides a mechanism to lower the well cleaning tool 110 into, and extract the well cleaning tool 110 from, the casing. A discharge head 220 may be located at the opposite end of the body 212. A conventional centralizer 216 may be attached adjacent the discharge head 220 to ensure that the discharge head is centrally located within the casing.

FIG. 3 is a perspective view of the discharge head 220. FIG. 4 is a cross sectional view of the discharge head 200. Referring now to both FIG. 3 and FIG. 4, the discharge head 220 may have a positive electrode 322 and a negative electrode 324 separated by a gap 326. When a high DC voltage is placed between the positive electrode 322 and the negative electrode 324, an electrical discharge may occur across the gap 326. The terms “positive electrode” and “negative electrode” refer to the polarity of the DC voltage applied to the respective electrode.

The positive electrode 322 may be surrounded by a first insulator 328 which provides electrical isolation between the positive electrode and other components of the discharge head 220. The first insulator 328 may be held by a first electrode mount 330, which couples the first insulator 328 to the body (not shown in FIG. 3; 212 in FIG. 4) of the well cleaning tool.

The negative electrode 324 may be held by a second electrode mount 332. The position of the negative electrode 324 may be adjustable, continuously or in steps with respect to the second electrode mount 332 to allow adjustment of a width of the gap 326 (i.e. the spacing between the negative electrode 324 and the positive electrode 322). Four elongate legs 334 (two of which are identified in each of FIG. 3 and FIG. 4) may extend from the second electrode mount 332 to couple the second electrode mount 332 to the first electrode mount 330. The use of four legs 334 is exemplary; a discharge head may use two or more legs to couple the first and second electrode mounts 330, 332. A cap 336 may affix the first electrode mount 330 and the second electrode mount 332 to the body 212 of the well cleaning tool.

Referring now only to FIG. 4, the negative electrode 324 may be surrounded by a second insulator 438 (not shown in FIG. 3). A plurality of O-ring seals 440 may be disposed between the positive electrode 322 and the first insulator 328, between the first insulator 328 and the first electrode mount 330 and between the first electrode mount 330 and the body 212. These O-ring seals may collectively prevent intrusion of fluids from the well casing into the interior of the body 212.

A third insulator may further electrically isolate the positive electrode 322 for the body 212. A first electrical connection 444 may connect the positive electrode 322 to the components within the body 212, which will be described subsequently. A second electrical connection 446 may connect the negative electrode 324 to the components within the body 212 via the first electrode mount 330, the legs 334, and the second electrode mount 332.

The first electrode mount 330 and the second electrode mount 332 may be fabricated from a metal material that is electrically conductive and provides sufficient mechanical strength to hold the electrodes in position during and after repeated electrical discharges. The first electrode mount 330, the second electrode mount 332, and the cap 336 may be fabricated, for example, from stainless steel, corrosion resistance steel, low carbon steel, or another sufficiently strong metal material.

FIG. 5 is a detailed cross-sectional view of portions of the discharge head 220. The positive electrode 322 may have a first end surface 548 separated from a second end surface 552 of the negative electrode 324 by a discharge gap 326. The positive electrode 322 and the negative electrode 324 may be generally cylindrical and may be coaxial, which is to say they may share a common axis 556. The axis 556 may be the axis of the generally cylindrical body 212 (not shown in FIG. 5). The first end surface 548 may be generally circular with a diameter d1 about 0.5 inch to 1.0 inch. The second end surface 552 may also be generally circular with a diameter d2 about 0.5 inch to 1.5 inch. The diameter d2 may be the same as or greater than d1. These dimensions are exemplary and larger or smaller electrodes may be used.

The first insulator 328 may surround the positive electrode 322. During operation of the well cleaning tool, electrical discharges between the first end surface 548 and the second end surface 552 may generate substantial shock waves in the fluid within the well. In this context, a shock wave may be considered “substantial” if the shock wave is effective to remove at least a portion of undesired deposits from a well casing surrounding the discharge head. A surface 550 of the first insulator may be shaped to direct the shock waves generally orthogonal to the axis 556. For example, the surface 550 may be a convex curved surface or a conical surface. The surface 550 may be substantially flush with the first end surface 548, which means the first end surface 548 is not significantly recessed below the surface 550 and does not significantly protrude above the surface 550. The surface 550 and the first end surface 548 are considered to be substantially flush if a deviation between the first end surface 548 and an extension of the surface 550 at the axis 556 is small compared to the diameter d1.

The second insulator 438 may surround the negative electrode 324. A surface 554 of the second insulator may also be shaped to direct the shock waves generally orthogonal to the axis 556. For example, the surface 554 may be a convex curved surface or a conical surface. The surface 554 may be substantially flush with the second end surface 552. The surface 554 and the second end surface 552 are considered to be substantially flush if a deviation between the second end surface 552 and an extension of the surface 554 at the axis 556 is small compared to the diameter d2.

A width w of the discharge gap 326 may be set by adjusting the position of the negative electrode 324 and the second insulator 438, if present, with respect to the second electrode holder 332 (not shown in FIG. 5).

The first insulator 328, the second insulator 438, and the third insulator 442 may be fabricated from a dielectric material that is compatible with the temperatures and fluids to be encountered with a well. A primary function of the first insulator 328 and the third insulator 442 is to isolate the first electrode from the other components of the well cleaning tool. A dielectric breakdown strength of the material used for the first and third insulators 328, 442 must be sufficient to withstand the voltage, which may exceed 10,000 volts, between the positive electrode 322 and the first electrode mount 330 when a discharge is initiated. A surface sheet resistivity of the material used for the first and third insulators 328, 442 must also be sufficient to prevent significant electrical conduction across a surface of these insulators between the positive electrode 322 and other components when a discharge is initiated. Other factors that may affect the selection of the material for the insulators include machinability, strength, and dimensional stability.

Materials that may be used for the first, second, and third insulators 328, 438, 442 include ceramics such as alumina and unfilled or filled polymer materials such as epoxy-glass laminates (G-10, FR-4, etc.), polyetherimide (sold under, for example, the ULTEM® and ULTEM® 2300 names), polyetheretherketone (PEEK), and polyoxymethylene (sold under, for example, the DELRIN® name). Other polymer and co-polymer materials may also be used. Filled polymer materials may be filled with, for example, 10% to 40% glass fiber.

Repeated arcs or electrical discharges between electrodes (for example, in arc welding) are known to cause erosion of the electrodes. To minimize erosion, electrodes are commonly made from a refractory material such as tungsten or graphite. The inventors of the well cleaning method, system, and tool described herein have found that erosion occurs primarily in the positive electrode. Further, experiments conducted by the inventors have shown that mounting the first end surface 548 of the positive electrode 322 substantially flush with the surface 550 of the first insulator 328 results in gradual erosion around a perimeter of the first end surface 548. This design allows the electrodes 322, 324 to be fabricated from a non-refractory material such as carbon steel, low alloy steel, stainless steel, or corrosion resistant steel.

FIG. 6 is a perspective view of a portion of a discharge head including the first end surface 548 of the positive electrode 322, the surface 550 of the first insulator 328, and the first electrode mount 330. The first electrode mount 330 may be configured with slots 658 to receive the legs 334 of the second electrode mount 332 (not shown in FIG. 6). The number of slots 658 will correspond with the number of legs 334 of the second electrode mount 332. As with the legs 334, two or more slots 658 may be included in first electrode mount.

FIG. 7 shows a block diagram of the well cleaning system 100, which includes a surface installation 102 and a well cleaning tool 110 linked by a cable 104. The surface installation 102 may commonly be housed in a truck, as illustrated in FIG. 1, but is not limited to that implementation. The surface installation may include a primary power supply 708, which may be, for example, a generator or batteries. The primary power supply 708 may provide primary power to the well cleaning tool 110 via the cable 104. The primary power may be AC or DC power. The primary power voltage may be several hundred volts or greater to minimize the effects of a voltage drop that will occur in the cable 104, which may extend as far as several miles into the well.

The surface installation 102 may also include instrumentation and control subsystem 706 to control and document the operation of the well cleaning tool. At a minimum, the instrumentation and control subsystem 706 may provide the ability to selectively enable operation of the well cleaning tool 110 when the tool is in a target region of the casing requiring cleaning and to selectively disable operation of the tool when in other regions of the casing. The instrumentation and control subsystem 706 may be configured to control operational parameters of the well cleaning system 100. For example, the instrumentation and control subsystem 706 may be configured to control one or more of the rate at which the tool descends and ascends in the casing, the rate or frequency of electrical discharges produced by the tool, the electrical voltage or energy of each discharge, and other operational parameters.

The instrumentation and control subsystem 706 may also store operational data and otherwise document the operation of the well cleaning system. For example, the instrumentation and control subsystem 706 may store or otherwise document the depth and time when the well cleaning tool was activated and the depth and time when the well cleaning tool 110 was deactivated. If appropriate feedback is received from the well cleaning tool 110, the instrumentation and control subsystem may store additional data such as, for example, a count of the number of electric discharges that occurred between activation and deactivation, the time and depth of some or all of the electrical discharges, the time duration and/or peak current of some or all of the electrical discharges, and other information pertaining to the operation of the well cleaning system and the environment in which it operates.

Control and feedback information may flow between the instrumentation and control subsystem 706 and the well cleaning tool 110 via the cable 104. The control and feedback information may flow via separate wires or optical fibers in the cable 104. Alternatively or additionally, control and feedback information may flow over the same wires used to convey the primary power. Information may be conveyed over the primary power wires using any of numerous techniques and standards developed for power line communications for applications such as utility grid monitoring and home networking.

The well cleaning tool 110 may include a power converter 762. The power converter 762 may receive primary power from the primary power supply via the cable 104 and may convert the primary power into DC power of sufficiently high voltage to create a discharge between electrodes 322 and 324. A variety of techniques and circuits may be used in the power converter. For example, the power converter may include a DC-AC inverter to convert DC primary power into a high frequency AC signal, a step-up transformer that accepts the AC signal and outputs a higher voltage AC signal, and a voltage multiplier that uses a combination of rectifiers and capacitors to convert the output of the step-up transformer into a high DC voltage level. For example, 200-volt DC primary power may be converted into a 200-volt high frequency AC signal. The 200-volt AC signal may be applied to a primary winding of a step-up transformer having a 1:15 turns ratio between its primary and secondary windings. The step-up transformer secondary winding may output a 3000-volt AC signal. The 3000-volt AC signal may be tripled in a voltage multiplier to provide a 9000 volt DC level. These voltages are only provided as examples; other voltages may be used in the power converter.

The well cleaning tool 110 may include an energy reservoir 764 fed by the output of the power converter 762. The energy reservoir 764 may be, for example, a high voltage capacitor or a plurality of capacitors connected in series and/or parallel to collectively function as a high voltage capacitor. The power converter 762 may be configured with a limited output current capacity, such that the energy reservoir 764 may be gradually charged from a discharged state to the full voltage output from the power converter. Once the energy reservoir 764 is charged to a desired voltage level, a switch 766 may connect the energy store to the electrodes 322 and 324, causing an electrical discharge that depletes the energy stored in the energy reservoir 764. The power converter 762 may then begin recharging the energy reservoir 764 in preparation for the next electrical discharge.

The switch 766 may be, for example, a triggered spark gap. The switch 766 may be a solid state switch using a cascade of semiconductor devices as described, for example, in U.S. Pat. No. 4,040,000. The switch 766 may be a gas-filled or vacuum tube device such as a thyratron or krytron. The switch 766 may be another device or combination of devices capable of both blocking the high voltage level produced by the power converter and passing very high instantaneous current each time the stored energy is discharged through the electrodes 322, 324.

The well cleaning tool 110 may include a controller 760. The controller may be configured to control the operation of the well cleaning tool and to periodically trigger the switch 766 to initiate a series of electrical discharges between the electrodes 322, 324. For example, the controller 760 may monitor voltage at the energy reservoir 764 during charging and trigger the switch 766 when the voltage at the energy reservoir 764 reaches a predetermined discharge level. The predetermined discharge level may be less than or equal to the maximum voltage output by the power converter 762. The predetermined discharge level may be greater than or equal to a minimum voltage sufficient to cause a discharge between the electrodes 322 and 324. The predetermined discharge level may be set in accordance with a desired power for each discharge (which will be generally proportional to the square of the discharge voltage level). The predetermined discharge level may be set in consideration of the conductivity of the fluid in the well and the diameter of the well casing. The discharge level may be preprogrammed into the controller on the surface before the well cleaning tool 110 is lowered into a well. The discharge level may be determined by the instrumentation and control subsystem 706 and communicated to the controller 760 in the well via the cable 104.

The controller 760 may be configured to selectively enable and disable the operation of the well cleaning tool 110 in response to commands received from the instrumentation and control subsystem via the cable 104. Alternatively, the operation of the well cleaning tool 110 may be enabled and disabled from the surface by selectively providing or not providing the primary power from the primary power supply 708.

The controller 760 may be configured to transmit feedback information to the instrumentation and control subsystem 706 via the cable 104. The controller 760 may monitor analog operational parameters of the well cleaning tool 110, digitize the analog data, and transmit the digitized data via the cable using a power line communication technique. For example, the controller 760 may monitor the voltage at the energy reservoir 764 and transmit data representative of the peak voltage prior to some or all discharges. The controller 760 may detect a voltage drop across a very low value resistor in series with one of the electrodes 322, 324 and transmit data representative of the peak current or current-versus-time waveform for some or all discharges. The controller 760 may be coupled to a sensor that detects the light generated by each discharge and transmit data representative of the peak light output or light output-versus-time waveform for some or all discharges. The controller 760 may be coupled to one or more sensors that detect the temperature at various locations or components within the well cleaning tool and transmit data representative of these temperatures. The controller 760 may transmit other operational data to the instrumentation and control subsystem.

As previously discussed, discharging the well cleaning tool in an oil environment can lead to deterioration of the insulating surfaces that electrically isolate the electrodes 322 and 324. To avoid this deterioration, the well cleaning tool 110 may incorporate a seal 768 to prevent fouling the electrodes and adjacent insulating surfaces with oil as the tool is lowered through the primarily oil-filled upper portion 188 that may be present at the top of a well. The seal 768 may be a retractable cover configured to seal the discharge head 220 or at least the surface of the first insulator 328 from the oil. The cover may be retracted to allow operation of the well cleaning tool 110 in the water-filled lower portion 186 of the well. The cover may be retracted, for example using a motor, a solenoid, a spring, or some other mechanism.

The seal 768 may be a consumable cover that is sealed over the discharge head 220 or at least the surface of the first insulator 328. For example, the seal 768 may be a preformed cover that is sealed over all or portions of the discharge head 220 using adhesive tape before the tool is lowered into a well. The consumable cover may be filled with water or another fluid to provide a medium about the electrodes 322, 324. The consumable cover may then be destroyed or otherwise dislodged from the tool by one or more shock waves resulting from discharging the well cleaning tool 110 after the well cleaning tool reaches the water-filled lower portion 186 of the well.

The seal 768 may be a consumable material coated over portions of the discharge head including at least the surface of the first insulator 328 before the tool is lowered into a well. The coating material may be impervious to oil and soluble in water, such that the coating dissolves when the well cleaning tool reaches the water-filled lower portion 186 of the well. The coating material may be, for example, a liquid soap or liquid detergent, which may also serve to emulsify any residual oil on the discharge head 220.

Description of Processes

Referring now to FIG. 8, a process 800 for cleaning an oil well may start at 805 when a well cleaning system, such as the well cleaning system 100, is made available at the site of the well. The process 800 may conclude at 895 after the well has been cleaned. The process 800 is a process for cleaning at least one target portion of an oil well of flow-restricting deposits using a well cleaning tool, such as the well cleaning tool 110, that discharges stored electric energy to generate shock waves effective to remove the deposits.

To avoid degradation, a cleaning tool, such as the well cleaning tool 110, may be inhibited from generating electrical discharges when the environment about the tool is primarily oil, or when a discharge head is contaminated with oil. Thus it may be necessary to determine a depth of an oil-water boundary within the well before cleaning the well. At 810 a survey of the well may be conducted by lowering one or more survey tools or instruments into the well. The survey at 810 may be performed by an operator of the well, by the party who will clean the well, or by a third-party oil field services contractor. The location of the oil-water boundary may be determined, for example, by lowering an appropriate tool into the well and measuring one or more of electrical resistivity or conductivity, opacity, viscosity, dielectric constant, inductance, capacitance, density differential, or some other parameter indicative of the fluid content of the well as a function of depth within the well.

Like most electronic equipment, a well cleaning tool may be subject to a maximum internal operating temperature, which will depend upon the temperature of the fluids in the well. Surveying the well at 810 may also include determining the temperature of the fluids in the well as a function of depth or a least at one or more target portions of the well to be cleaned. Additionally, surveying the well at 810 may include a caliper measurement or other casing assessment survey or video inspection of the well to locate any severely corroded or otherwise compromised portions of the casing. To avoid further degradation of compromised portions of the casing, such portions may not be subjected to the cleaning process. A video inspection at 810 may also determine the nature and extent of the deposits to be removed by the well cleaning process.

If the results of the survey at 810 indicate that the oil-water boundary is at or below a target portion of the well to be cleaned, water may be added to the well (and oil necessarily extracted from the top of the well) at 820 to raise the oil-water boundary above all of the at least one target portion to be cleaned. Adding water to the well at 820 or continuously during the cleaning process may be an effective method to cool the fluids in the well if the well survey at 810 indicates that the native fluids in the well are too hot to allow operation of the cleaning tool.

At 830, operating parameters for the well cleaning tool may be determined. Operating parameters may be based, at least in part, on results of the survey performed at 810. The operating parameters may include a minimum operational depth to which the well cleaning tool must be lowered before stored energy can be discharged without incurring degradation. The minimum operational depth may be based on the depth of the oil-water boundary from 810 and the amount of water, if any, added to the well at 820. The operating parameters may include a discharge repetition rate and/or a maximum duration of operation based on the temperature of the fluid in the well at each target portion to be cleaned. The operating parameters may include a speed at which the well cleaning tool is lowered through each target portion of the casing while repeatedly discharging to generate shock waves. The operating parameters determined at 830 may include an energy per discharge level based on an inside diameter of each target portion of the well casing to be cleaned and/or the nature and extent of the deposits seen during video inspection of the well at 810.

The operating parameters determined at 830 may include a spacing of the electrodes in the discharge head based on the electrical conductivity or salinity of the fluids in the well. The spacing determined for the electrodes may be generally inverse to the conductivity of the fluids in the well. The electrodes may be closely spaced if the fluids in the well have low conductivity and the electrodes may be spaced further apart if the fluids are highly conductive.

As previously discussed, deterioration may occur if the well cleaning tool is discharged when oil is present within the tool discharge head. To avoid this deterioration, all or a portion of the discharge head may be sealed at 840 to prevent fouling the discharge head with oil as the tool is lowered through the oil layer above the oil-water boundary in the well. Sealing the discharge head at 840 may include positioning a retractable cover configured to seal the discharge head or at least an insulating surface within the discharge head from the oil.

Sealing the discharge head at 840 may include installing a consumable cover configured to seal the discharge head or at least an insulating surface within the discharge head from the oil. For example, the seal installed at 840 may be a preformed cover that is sealed over all or portions of the discharge head using adhesive tape before the tool is lowered into a well. The consumable cover may be filled with water or another fluid to provide a medium in which an initial discharge may occur.

Sealing the discharge head at 840 may include applying a consumable coating over portions of the discharge head including at least the surface of an insulator within the discharge head before the tool is lowered into a well. The coating may be a material impervious to oil and soluble in water, such that the coating dissolves when the well cleaning tool reaches the water-filled portion of the well. The coating material may be, for example, a liquid soap or liquid detergent, which may also serve to emulsify any residual oil on the discharge head.

At 850, the well cleaning tool may be lowered through the oil-water boundary into a primarily water-filled portion of the well. After the well cleaning tool passes below the oil-water boundary, the discharge head may be unsealed at 860 to allow operation of the well cleaning tool in the water-filled portion of the well. In the case where the discharge head is sealed by a retractable cover, at 860 the cover may be retracted, for example using a motor, a solenoid, a spring, or some other mechanism. In the case that the discharge head is sealed by a consumable cover, the consumable cover may be destroyed or otherwise dislodged from the tool at 860 by one or more shock waves resulting from discharging the well cleaning tool. In the case that the discharge head was sealed using a consumable coating, at 860, the coating may dissolve in the water that fills the well below the oil-water boundary.

After the discharge head is unsealed at 860, the target portion or portions of the well may be cleaned at 870. Cleaning may be performed by lowering or raising the tool through each target portion while repeatedly discharging stored energy to generate shock waves. Each target portion may be cleaned by a single upward or downward pass of the well cleaning tool. Each target portion may be cleaned by multiple upward and/or downward passes of the well cleaning tool. During cleaning, the well cleaning tool may be operated in accordance with the operating parameters defined at 830.

After the cleaning has been completed at 870, the well cleaning tool may be deactivated at 880 to ensure that the tool does not discharge stored energy while it is being withdrawn from the casing at 890. For example, the well cleaning tool may be deactivated at 880 by a command sent from a control subsystem on the surface to a controller within the tool. The well cleaning tool may be deactivated at 880 by discontinuing the delivery of power from the surface to the tool. The process 800 may end at 895 after the tool has been withdrawn from the well.

Closing Comments

Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items. 

It is claimed:
 1. A well cleaning tool, comprising: an elongate housing configured to be lowered into a well casing; a discharge head disposed at one end of the housing, the discharge head comprising: a positive electrode having a first end surface, a first insulator surrounding the positive electrode, a surface of the first insulator substantially flush with the first end surface, and a negative electrode having a second end surface separated from the first end surface by a discharge gap; an energy reservoir disposed within the housing; and a switch disposed within the housing, the switch configured to selectively couple the energy reservoir to the positive electrode and the negative electrode to cause energy stored in the energy reservoir to discharge across the discharge gap.
 2. The well cleaning tool of claim 1, further comprising: a power converter disposed within the housing, the power converter configured to convert primary electrical power into a DC voltage level to charge the energy reservoir.
 3. The well cleaning tool of claim 1, further comprising: a first electrode mount configured to couple the first insulator to the housing.
 4. The well cleaning tool of claim 3, further comprising: a second electrode mount configured to retain the negative electrode, the second electrode mounting including three or more elongate legs to couple the second electrode mount to the first electrode mount.
 5. The well cleaning tool of clean 4, wherein the second electrode mount is configured to retain the negative electrode in a plurality of positions to provide for adjustment of a width of the discharge gap.
 6. The well cleaning tool of claim 1, wherein the positive electrode, the negative electrode and the housing are generally cylindrical in shape and disposed along a common axis.
 7. The well cleaning tool of claim 6, further comprising: a centralizer adjacent to the discharge head, the centralizer configured to center the discharge head within the well casing.
 8. The well cleaning tool of claim 6, wherein the surface of the first insulator is configured to direct a shock wave generated by the discharge generally orthogonal to the common axis.
 9. The well cleaning tool of claim 8, wherein the surface of the first insulator is one of a convex curved surface and a conical surface.
 10. The well cleaning tool of claim 6, further comprising: a second insulator surrounding the negative electrode, a surface of the second insulator substantially flush with the second end surface.
 11. The well cleaning tool of claim 10, wherein the surface of the second insulator is configured to direct the shock wave generated by the discharge generally orthogonal to the common axis.
 12. The well cleaning tool of claim 11, wherein the surface of the second insulator is one of a convex curved surface and a conical surface.
 13. The well cleaning tool of claim 1, wherein the positive electrode and the negative electrode are one of carbon steel, low alloy steel, stainless steel, and corrosion resistant steel. 